Described in 1858, the partial skeleton of Hadrosaurus foulkii was one of the most important dinosaur discoveries ever made. At that time, the few known dinosaurs were represented by a collection of scraps—paltry fragments that allowed paleontologists to reconstruct them first as giant lizards, and then as strange quadrupedal beasts. The elements of Hadrosaurus caused naturalists to revise what they thought dinosaurs looked like. Among the remains of Hadrosaurus pulled from a New Jersey marl pit were the arms and legs, and the difference in their lengths caused scientists to realize that this dinosaur could have walked on its hind limbs alone. Tracks discovered in England of a dinosaur walking bipedally, the 1866 discovery of the tyrannosaur Dryptosaurus in southern New Jersey, and Thomas Henry Huxley's notion that dinosaurs were very bird-like all supported this conclusion, and spurred a rapid re-imagining of what dinosaurs looked like.
Strangely, though, Hadrosaurus has fallen from grace since the time of its description. It may have helped revolutionize the image of dinosaurs held by 19th century naturalists, but during the past century and a half no one has ever found a more complete skeleton. Nor is anyone likely to find one. The site where Hadrosaurus was discovered has been turned into a suburban development—a plaque on a rock commemorates the find—and the only major Cretaceous fossil site still being investigated in New Jersey is slightly geologically younger than the one where Hadrosaurus was found. This has presented paleontologists with a problem. Without more complete remains, how can we be sure that the bones of Hadrosaurus represent a distinct dinosaur and are not just pieces of some more completely known species found elsewhere?
A 2006 paper by Albert Prieto-Márquez, David Weishampel and Jack Horner cast doubt on the status of Hadrosaurus. In a reevaluation of the skeleton, they could not find any distinguishing characteristics. Even though the whole group of dinosaurs was named after it, there was no definite way to tell what sort of hadrosaur Hadrosaurus was.
But Prieto-Márquez has now changed his mind. In a paper just published in Zootaxa, he has concluded that the paltry remains of that first Hadrosaurus skeleton contain some diagnostic characteristics, after all. While the proportions of the upper arm bone, the humerus, are like those of the related iguanodont dinosaurs, Hadrosaurus has some minute specializations of the hip that differentiate it from almost all other hadrosaurs. A complete skeleton would be even more useful in this regard, but barring that, Prieto-Márquez has found a way to distinguish Hadrosaurus from similar dinosaurs. For now, New Jersey's official state dinosaur is safe.
Albert Prieto-Márquez (2011). Revised diagnoses of Hadrosaurus foulkii Leidy, 1858 (the type genus and species of Hadrosauridae Cope, 1869) and Claosaurus agilis Marsh, 1872 (Dinosauria: Ornithopoda) from the Late Cretaceous of North America Zootaxa, 2765, 61-68
Albert Prieto-Márquez, David B. Weishampel, and John R. Horner (2006). The dinosaur Hadrosaurus foulkii, from the Campanian of the East Coast of North America, with a reevaluation of the genus Acta Palaeontologica Polonica, 51 (1), 77-98